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| United States Patent |
6,014,537
|
|
Van Aken
, et al.
|
January 11, 2000
|
Method of developing an image in an image forming apparatus
Abstract
An image forming apparatus is used, in which an electrostatic image formed
on a moving image forming belt is developed by AC development. Where the
image forming belt moves at a speed of v.sub.p mm/s, the cleaning
potential is V.sub.cl volts, and the AC bias frequency is f kHz, the
function Z satisfies the following equation:
##EQU1##
Images substantially free of background development are thereby obtained.
| Inventors:
|
Van Aken; Luc Karel Maria (Hasselt, BE);
Janssens; Robert Frans Louisa (Geel, BE)
|
| Assignee:
|
Xeikon NV (Mortsel, BE)
|
| Appl. No.:
|
295442 |
| Filed:
|
April 21, 1999 |
Foreign Application Priority Data
| Current U.S. Class: |
399/271; 430/122 |
| Intern'l Class: |
G03G 015/09; G03G 013/22 |
| Field of Search: |
399/240,241,291,293,295,270,271,285
430/122
|
References Cited
U.S. Patent Documents
| 4844008 | Jul., 1989 | Sakemi et al. | 399/270.
|
| 5185496 | Feb., 1993 | Nishimura et al. | 399/270.
|
| 5227270 | Jul., 1993 | Scheueret et al.
| |
| 5314774 | May., 1994 | Camis.
| |
| 5409791 | Apr., 1995 | Kaukeinen et al. | 430/122.
|
| 5631679 | May., 1997 | Kagayama | 399/291.
|
| 5701553 | Dec., 1997 | Endo et al. | 399/291.
|
| Foreign Patent Documents |
| 0 432 998 A2 | Jun., 1991 | EP.
| |
Other References
Kobayashi et al., "A Mechanism for Resolution Decrease in Organic
Photoconductor", Journal of Imaging Science and Technology, vol. 39. No.
6, Nov./Dec. 1995, pp. 485-489.
Nguyen et al., "Technology Trends in the Development of Organic
Photoconductors for New Applications", Journal of Imaging Technology 15;
pp. 158-163, 1989.
Ishikawa et al., "Organic Photoconductors for Electrophotography",
Reprographic Products Group, pp. 82-97.
Weiss et al., "Photofatigue of Organic Photoreceptors with
Triarylamine-Based Charge Transport Layers", Journal of Imaging Science
and Technology, vol. 39, No. 5, Sep./Oct. 1995, pp. 425-428.
Baumann et al., "Organic Semiconductors Used in Electrophotography", IS&T
Reporter, vol. 8, No. 2, Jun. 1993.
|
Primary Examiner: Lee; Susan S. Y.
Attorney, Agent or Firm: Larson & Taylor
Claims
We claim:
1. A method of developing an electrostatic image formed on a moving image
forming belt having a photoconductive surface and a back electrode, by
charging the photoconductive surface to a dark potential V.sub.0 (volts) to
form a charged photoconductor surface,
image-wise exposing said charged photoconductor surface to form an
electrostatic image thereon,
moving said image forming belt with a belt speed v.sub.p (mm/s) to bring
said electrostatic image into the vicinity of a developing device in which
a magnetic brush of developer material is established,
applying a DC developing bias potential V.sub.DC between said magnetic
brush and said back electrode, with a cleaning potential V.sub.cl (volts),
and
superimposing an AC voltage having an AC bias frequency f (kHz) over said
DC developing bias potential, wherein said cleaning potential V.sub.cl,
said AC bias frequency f and said belt speed v.sub.p are such that:
##EQU5##
2. The method of claim 1, wherein said cleaning potential V.sub.cl, said AC
bias frequency f and said belt speed v.sub.p are such that:
3. The method of claim 1, wherein said cleaning potential V.sub.cl lies
between 20 and 250 volts.
4. The method of claim 1 wherein said AC bias frequency f lies between 1
and 8 kHz.
5. The method of claim 1 wherein said speed of said image forming belt
v.sub.p lies between 50 and 500 mm/s.
6. The method of claim 1, wherein said electrostatic image is developed by
reversal development and said dark potential V.sub.0 lies between 200 and
800 volts.
Description
FIELD OF THE INVENTION
The present invention relates to a method of using an image forming
apparatus, such as a copier, printer or the like, in which an
electrostatic image is formed on an image forming member, from which it is
subsequently transferred, directly or indirectly to a substrate.
BACKGROUND TO THE INVENTION
In a typical image forming apparatus, an electrostatic image is formed on
an image forming member, which may for example be the photoconductive
surface of a rotating drum or the photoconductive surface of a moving
belt. The electrostatic image is, for example, formed by charging the
photoconductive surface to a first potential V.sub.0, known as the "dark"
potential, and then image-wise exposing the charged photoconductor surface
to dissipate the charge on image areas. The electrostatic image is brought
into the vicinity of a developing device, which is supplied with
developer, typically a mixture of a particulate toner and magnetic carrier
particles.
It is common practice to apply the toner-carrier mixture to the surface
carrying the electrostatic charge image by means of a developing unit
wherein toner and magnetizable carrier particles are mixed and a layer of
the toner-carrier mixture, referred to herein as "developer", is picked up
by an applicator such as a rotating sleeve or drum having magnets inside,
forming a so-called magnetic brush on a "magnetic roller".
In one type of development unit toner particles are mixed with larger
magnetizable carrier particles, to which the toner particles adhere by
electrostatic attraction force. The electrostatic charge of the toner and
carrier particles is obtained triboelectrically by agitation. The charge
sign of the toner particles is opposite to the charge sign of the carrier
particles.
On rotating the magnetic roller, the toner particles still adhering to the
magnetically attracted carrier particles are brought into a developing
zone wherein the toner particles are separated from the carrier particles
by the electrostatic attraction forces of the electrostatic latent image
to be developed and transfer to the latent electrostatic charge image. The
sign of the toner particles, compared with the sign of the charge on the
image forming member, determines whether the development is a "direct" or
"reversed" development. If the toner and the image forming member have
opposite signs, the development is direct; toner particles will be
attracted to the charged areas of the image forming member. If the toner
and the image forming member have the same sign, the development is
"reverse"; toner particles will be attracted to the discharged areas of
the image forming member.
A DC developing bias potential V.sub.DC of suitable value is applied
between the magnetic brush and the back electrode of the image forming
member. The sign of the DC bias potential is the same as that of the image
forming member. The value of the DC bias potential is typically between
the value of the potential of the image areas and that of the non-image
areas.
The term "cleaning potential" is defined as the absolute value of the
difference between the potential of the non-image areas and the DC bias
potential. The main effect of this cleaning potential is to establish an
electric field between the magnetic roller and the image forming member at
the non-image areas which repulses the toner particles away from the image
forming member back to the magnetic brush.
The term "development potential" is defined as the absolute value of the
difference between the potential of the image areas and the DC bias
potential. The main effect of this development potential is to establish
an electric field between the magnetic roller and the image forming member
at the image areas which attracts the toner particles to the image areas.
Toner particles are attracted to the electrostatic image on the image
forming member to thereby form a toner image. Subsequently the image
forming member, carrying the toner image, comes into contact with a
substrate, for example paper in sheet or web form, to which the toner
image is transferred. Alternatively, the transfer of the toner image from
the image carrying member to the substrate may be by way of one or more
intermediate transfer members.
It is known to superimpose an AC voltage over the DC bias between the
developer carrying member and the back electrode of the image forming
surface.
This AC development method has a number of advantages. Higher toner amounts
can be transferred towards the photoconductor during AC development than
can be achieved with DC-only development, resulting in higher print
densities on the image. Using an AC electric field during development
reduces the development time constant considerably, resulting in a better
development of image areas containing a sharp transition from a high
density to a low density or vice versa. The result is an image with
sharper well-defined image edges. The image density developed with AC
development is less sensitive to variations in distance of the
photoconductor to the magnetic roller, and less sensitive to variations in
developer supply on the magnetic roller. Furthermore, AC development leads
to images with less blow-off and a better homogeneity of line widths.
An example of an image forming apparatus using AC development is shown in
U.S. Pat. No. 5,314,774 (Hewlett Packard) which describes a method and
apparatus for developing and printing color images on a moving
photoconductive belt. A number of developing devices are spaced from the
belt and are AC and DC biased to project toner onto the belt. The
composite color image thereby formed on the belt is then transferred to an
intermediate belt and from there to a final substrate. A relationship is
disclosed defining the motion of toner particles in the air gap between
the developer carrying member in the developing device, and the belt in
terms of the size of the toner particles, the viscosity of the air gap,
the charge on the toner and the DC and AC electrostatic fields.
European patent specification EP 432998-A (Xerox Corporation) describes a
scavenge-less/non-interactive electrostatographic development system in
which a powder cloud is generated between a developer donor roller and a
set of wires mounted between the donor roller and an image forming belt.
For use in highlight color imaging, the system uses the combination of an
AC voltage on the donor roller with an AC voltage between the toner cloud
forming wires and the donor roller to control the developability of lines
and the degree of interaction between the toner and a photoconductive
belt.
U.S. Pat. No. 5,409,791 (Eastman Kodak Company) describes an image forming
method in which an electrostatic image on an photoconductive belt already
carrying a loose dry first toner image is toned with a second toner, for
example having a different color. The toning is accomplished by a
developer having a high coercivity permanently magnetized carrier and
toner which is moved through a development zone by a rapidly rotating core
inside a sleeve on which the developer moves. Scavenging of the first
toner image is prevented by separating the sleeve from the photoconductive
belt sufficiently that the crests of the developer do not touch the
photoconductive belt during the toning process. An alternating electrical
field is applied between the sleeve and the photoconductive belt to
enhance development.
A problem which arises with AC development onto photoconductor belts,
especially where the photoconductor is an organic photoconductor, is
background development, especially when AC development is used in
combination with a high belt speed. It appears that the higher surface
roughness which is typical of belt photoconductors, as compared with drum
photoconductors, contributes to this problem.
It is an object of the present invention to provide a method of AC
development, in which the image is substantially free of background.
SUMMARY OF THE INVENTION
We have discovered that this objective and other useful benefits can be
achieved where the dark potential, the DC bias potential, the AC bias
frequency, and the belt speed satisfy a specified relationship.
Thus according to the invention, there is provided a method of using an
image forming apparatus, in which an electrostatic image formed on a
moving image forming belt is developed by AC development, wherein the
function Z satisfies the following equation:
##EQU2##
where V.sub.cl is the cleaning potential in volts, f is the AC bias
frequency in kHz, and v.sub.p is the speed of the image forming belt in
mm/s.
Preferably, Z is at least 0.8. We have calculated that given the data
provided in U.S. Pat. No. 5,409,791 referred to above, the value of the
function Z lies between about 0.03 and 0.3. Insufficient data is provided
in EP 432998 (Xerox Corporation) referred to above to derive a value for
the function Z.
The present invention will usually involve forming a layer of developer on
a developer carrying member behind which a magnetic field generating
device is disposed. The developer is carried to a developing position
where the developing carrying member and the image bearing belt are
opposed. It is an important aspect of the present invention that the
developer layer contacts the image forming belt in the developing
position. This is in contrast to the arrangement described in U.S. Pat.
No. 5,409,791, where the developer sleeve is separated from the
photoconductive belt sufficiently to prevent scavenging of the first toner
image. In the present invention, the image forming belt does not already
carry a toned image when it arrives at the developing position, so that
the problem of scavenging described in U.S. Pat. No. 5,409,791 cannot
arise. In contrast to the method described in U.S. Pat. No. 5,409,791, the
"contact" development process of the present invention is able to lead to
better development, in particular to better image quality (reduced edge
effects, fewer problems with high-to-low and low-to-high image density
transitions, better grey level rendition and a lower image noise level)
and fewer problems with carrier loss.
The developer which is used in the method according to the invention
preferably comprises toner particles and non-permanently magnetized
magnetic carrier particles. Permanently magnetized carrier particles are
less preferred since they stick together and developers containing such
particles are difficult to mix and to charge, it is difficult to mix newly
added toner with such carrier particles and the developers exhibit very
bad flow characteristics. As a consequence developing units, such as
described in U.S. Pat. No. 5,409,791 referred to above, which use
developers containing permanently magnetized carrier particles consume a
lot of energy.
The toner particles preferably contain a mixture of a resin, a dye or
pigment of the appropriate color and normally a charge-controlling
compound giving triboelectric charge to the toner. In dual-component
developers which are normally used, carrier particles are also present for
charging the toner particles by frictional contact therewith. The carrier
particles may be made of a magnetizable material, such as iron or iron
oxide. Developing technologies other than magnetic brush development, such
as mono-component developers, can be used.
Dry-development toners essentially comprise a thermoplastic binder
consisting of a thermoplastic resin or mixture of resins including
coloring matter, e.g. carbon black or coloring material such as finely
dispersed pigments or dyes.
The mean diameter of dry toner particles for use in magnetic brush
development is conventionally about 10 .mu.m (ref. "Principles of Non
Impact Printing" by Jerome L. Johnson--Palatino Press Irvine Calif., 92715
U.S.A. (1986), p. 64-85). For high resolution development, the mean
diameter may be from 1 to 5 .mu.m (see e.g. British patent specification
GB-A-2180948 and International patent specification WO-A-91/00548).
However, in the present invention, the toner particle size may be from 5
to 15 .mu.m, most preferably between 7 and 12 .mu.m.
The toner particles contain in the resinous binder one or more colorants
(dissolved dye or dispersed pigment) which may be white or black or has a
color of the visible spectrum, not excluding however the presence of
infra-red or ultra-violet absorbing substances.
The thermoplastic resinous binder may be formed of polyester, polyethylene,
polystyrene and copolymers thereof, e.g. styrene-acrylic resin,
styrene-butadiene resin, acrylate and methacrylate resins, polyvinyl
chloride resin, vinyl acetate resin, copoly(vinyl chloride-vinyi acetate)
resin, copoly(vinyl chloride-vinyl acetate-maleic acid) resin, vinyl
butyral resins, polyvinyl alcohol resins, polyurethane resins, polyimide
resins, polyamide resins and polyester resins. Polyester resins are
preferred for providing high gloss and improved abrasion resistance. The
volume resistivity of the resins is preferably at least 10.sup.13
.OMEGA.-cm.
We prefer to use toners having a composition comprising a thermoplastic
binder together with from 10% to 50% by weight of a pigment, based on the
weight of the toner composition. The use of toner compositions having a
higher level of pigment therein enables images with a higher density to be
printed. Alternatively, for the same image density, smaller toner
particles can then be used.
The charge on the toner particles generated usually by an agitator in the
developing unit, preferably lies between 5 and 25 .mu.C/g, most preferably
from 10 to 20 .mu.C/g.
The magnetic brush, from which toner particles are removed during each
revolution, to be taken up by the developed electrostatic charge image,
has to be supplied with fresh toner-carrier mixture. This is normally done
by an agitator projecting or scooping up toner-carrier mixture onto the
magnetic roller from a housing for holding the developer. The partly
exhausted developer is returned to the bulk of developer contained in the
housing and has to be thoroughly mixed timely with freshly added toner to
keep the toner-carrier weight ratio within acceptable limits for obtaining
consistent development results.
Preferably, the applicator comprises a rotatable developing sleeve having
magnets located therein for attracting developer onto the sleeve.
The cleaning potential V.sub.cl preferably lies between 20 and 250 volts,
most preferably between 100 and 150 volts. If the cleaning potential is
too high, carrier particles may be attracted to the image forming member
resulting in carrier loss and/or breakdown. If the cleaning potential is
too low, the non-image areas will be soiled by background development.
The development potential V.sub.DEV preferably lies between 50 and 500
volts, most preferably between 150 and 350 volts. If the development
potential is too high, too many toner particles will be developed
resulting in a too high image density and in excessive toner consumption.
If the development potential is too low, insufficient development takes
place.
The absolute value of the dark potential V.sub.0 preferably lies between
200 and 800 volts, most preferably between 300 and 500 volts. If the
absolute value of the dark potential is too high, charge breakdown may
occur. If the absolute value of the dark potential is too low, the
development and cleaning potentials may be insufficient.
The preferable ranges for the DC bias potential V.sub.DC and the potential
after exposure, V.sub.e, are defined by the preferred ranges for the
cleaning potential V.sub.cl, the development potential V.sub.DEV and the
dark potential V.sub.0, since the following relations hold:
##EQU3##
The AC bias frequency f preferably lies between 1 and 8 kHz, most
preferably between 2 and 6 kHz. If the AC bias frequency is too high, high
bias currents are needed. Moreover, the advantages of AC development will
be lost because the toner particles stop being influenced by the AC
electric field because acceleration forces acting on the toner particles
will become too high. If the AC bias frequency is too low, the toner
particles will be able to follow each individual AC bias pulsation
resulting in a rippling effect in the developed image.
The AC peak-to-peak voltage V.sub.AC preferably lies between 500 and 3000
volts, most preferably between 1000 and 2000 volts peak-to-peak. If the AC
peak-to-peak voltage is too high, high bias currents are needed, charge
breakdown may occur and carrier loss may result. If the AC peak-to-peak
voltage is too low, the effect of AC bias development will be too small
and the corresponding advantages will not be attained.
The speed of the image forming belt vp preferably lies between 50 and 500,
most preferably between 125 and 300 mm/s. If the belt speed is too high,
development will be insufficient and more than one magnetic roller and/or
a magnetic roller with a large diameter will have to be used. If the belt
speed is too slow, the engine will have an undesirable low throughput.
The image forming belt may be in the form of a charge carrying belt onto
which charge images are deposited by ion-deposition or, more preferably,
in the form of a photoconductive belt. The photoconductive belt may
comprise a base layer of a polymer material of 60 to 200 .mu.m thickness
covered with a thin conductive layer as a back electrode (preferably 0.05
to 1 .mu.m thickness). If the overall thickness of the belt is too high,
the belt may be insufficiently flexible to closely follow the
circumference of guide rollers and may become subject to deformation on
standing. One or more layers of an inorganic photoconductor, or more
preferably an organic photoconductor, are positioned on top of the
conductive layer with a total thickness of, for example, from 10 to 20
.mu.m. To make contact with the back electrode, the belt has at least one
strip of conductive material positioned beyond the image area and
extending through the photoconductive layer. Conductive grounding brushes
may be provided to contact this conductive strip.
The apparatus may be in the form of a multi-color duplex printer of the
type comprising two image forming stations positioned one on either side
of a substrate path. Sheets to be printed, preferably removed from a stack
located within a housing of the apparatus, are fed along the path into
operational positions relative to the two image-forming stations where
toner images are transferred thereto and then to a fuser station where the
toner images are fixed.
The removed sheet may be fed through an alignment station which ensures the
longitudinal and lateral alignment of the sheet, prior to its start from
said station under the control of the imaging system. As the sheet leaves
the alignment station, it preferably follows a straight horizontal path
through the printer. The speed of the sheet, along the path, may be
determined by a driven pressure roller pair.
A buffer station may be positioned between the second image forming station
and the fuser station, allowing the speed of the sheet to decrease to
enable the speed of fuser to be lower than the speed of image formation.
Each image forming station comprises an endless image forming belt guided,
for example, over a plurality of idler guide rollers to follow a path to
advance successive portions of the image forming surface sequentially
through various processing stations disposed along the path of movement
thereof. The image forming surface of the belt is ideally positioned at
the outside of its loop. Drive means are provided for driving the belt,
preferably at a uniform speed and for controlling its lateral position.
The drive means for the belt may comprise one or more drive rollers,
driven by a controlled drive motor, to ensure a constant drive speed.
In a preferred embodiment, a portion of photoconductive belt passes through
a charging station which charges the belt to a substantially uniform
potential. Next, the belt passes to an exposure station which exposes the
photoconductive belt to successively record four latent color separation
images. The latent images are developed for example with magenta, cyan,
yellow and black developer material, respectively. These developed images
are transferred to the print sheet in superimposed registration with one
another to form a multicolor image on the sheet. After an electrostatic
latent image has been recorded on the image forming belt, the belt
advances this image to a development station which includes four
individual developer units.
Each developer unit may be of the type generally referred to in the art as
"magnetic brush development units". Typically, a magnetic brush
development system employs a magnetizable developer material including
magnetic carrier granules having toner particles adhering
triboelectrically thereto. The developer material is continuously brought
through a directional flux field to form a brush of developer material.
The developer particles are continuously moving so as to provide the brush
consistently with fresh developer material. Development is achieved by
bringing the brush of developer material into contact with the image
forming surface. The developer units respectively apply toner particles of
a specific color which corresponds to the compliment of the specific
color-separated electrostatic latent image recorded on the image forming
surface. The color of each of the toner particles is adapted to absorb
light within a preselected spectral region of the electromagnetic wave
spectrum. Each of the developer units is moved into and out of an
operative position. In the operative position, the magnetic brush is
closely adjacent to the image forming belt, whereas in the non-operative
position, the magnetic brush is spaced therefrom. During development of
each electrostatic latent image only one developer unit is in the
operative position, the remaining developer units being in their
non-operative position. This ensures that each electrostatic latent image
is developed with toner particles of the appropriate color without
inter-mingling.
Each development unit may include a magnetic roller. The moving image
forming belt moves close to, but not in contact with, the magnetic roller.
Spacing means, such as a fixed sliding backing shoe, may be provided to
determine a constant distance between the image forming surface of the
belt and the magnetic roller. The controlled DC+AC potential is applied
between the magnetic roller and the back electrode of the image forming
surface of the belt. The development unit may include or be associated
with a control device for setting the cleaning potential V.sub.cl within a
desired range and setting the AC frequency to ensure that the value of the
function Z exceeds 0.65.
After their development, the images are moved to toner image transfer
stations where they are transferred on a sheet of support material. At
each transfer station, the sheet follows the path into contact with the
image forming belt. The sheet is advanced in synchronism with the movement
of the belt. After transfer of the four toner images, the belt is cleaned
in a cleaning station. Thereafter, a lamp illuminates the belt to remove
any residual charge remaining thereon prior to the start of the next
cycle.
The timing of exposure of the four distinct images, the relative position
of these images on the image forming belt and the lengths of the path of
this belt between the successive transfer stations are such that as a
sheet follows the path through these stations, the partly simultaneous
transfer of the distinct toner images to the paper sheet is such that a
perfect registering of these images is obtained.
The buffer station may be provided with an endless transport belt which
transports the sheet bearing the color images to the fuser station. The
fuser station operates to melt the toner particles transferred to the
sheets in order to affix them. This operation requires a certain minimum
time since the temperature of the fuser is subject to an upper limit which
must not be exceeded. Otherwise the lifetime of the fuser roller becomes
unsatisfactory. For this reason, the speed of the fuser station may be
limited. It is advantageous to use a high speed of image formation and
image transfer, since the four color separations of each color image are
recorded by exposure station in succession, which means that the recording
time of one color image amounts to at least four times the recording time
of one color component. Therefore, a relatively high speed of the image
forming belt is required, and thus of the synchronously moving sheets, as
compared with a maximum usable traveling speed through the fuser station.
Furthermore, it may be desirable to adjust the fusing speed independently
of the image processing speed, i.e. the belt speed, for obtaining optimum
results. It should be noted that the image processing speed in the imaging
stations is preferably constant. The length of the buffer station should
be sufficient for receiving the largest sheet size to be processed in the
apparatus. The buffer station operates initially at the speed of the image
forming belts of image forming stations. The speed of this station is
reduced to the processing speed of the fuser station as the trailing edge
of the sheet leaves the second image forming station.
The fusing station can be of known construction, and can be arranged for
radiation or flash fusing, for fusing by convection and/or by pressure,
etc. Hot roller fusing is preferred.
One image-forming station need not necessarily operate with one exposure
station but may include more than one exposure station, each such station
co-operating with several developer units.
The printing apparatus is not limited to color reproduction but may also be
a black-and-white printer.
The printing apparatus is not limited to duplex printing but may also be a
single-side printer.
BRIEF DESCRIPTION OF THE DRAWING
The invention will be described in further detail, purely by way of
example, with reference to the accompanying drawings, in which:
FIG. 1 is a diagrammatic representation of one embodiment of an
electrophotographic duplex color printer;
FIG. 2 is an isometric view of one embodiment of a development unit of the
printer shown in FIG. 1;
FIG. 3 shows detail from part of the development unit shown in FIG. 2.
DETAILED DESCRIPTION
FIG. 1 shows a diagrammatic representation of one embodiment of an
electrophotographic duplex color printer.
The printer comprises a light-tight housing 10 which has at its inside a
stack 12 of sheets to be printed and loaded on a platform 13. The height
of this platform 13 is adjusted in accordance with the size of the stack
12. At its output the printer has a platform 14 onto which the printed
sheets are received.
A sheet to be printed is removed from stack 12 by a dispensing mechanism 15
of known construction for removing the top sheet from stack 12.
The removed sheet is fed through an alignment station 16 which ensures the
longitudinal and lateral alignment of the sheet, prior to its start from
said station under the control of the imaging system. As the sheet leaves
the alignment station, it follows a straight horizontal path 17 up to
output section 18 of the printer. The speed of the sheet, upon entering
said path, is determined by driven pressure roller pair 47, driven by a
stepper motor, the frequency of which is adjustable with an accuracy of a
piezo crystal (i.e. better than 10.sup.-6).
A number of processing stations are located along the path 17. A first
image-forming station 20 indicated in a dash-and-dot line is provided for
applying a multi-color image to the obverse side of the sheet and is
followed by a second station 21 for applying a multi-color image to the
reverse sheet side. A buffer station 23 then follows, with an endless
transport belt 24 for transporting the sheet to a fuser station 25 while
allowing the speed of the sheet to decrease because the speed of fuser 25
is lower than the speed of image formation.
Both image forming stations 20 and 21 being similar to each other, only
station 20 will be described in more detail hereinafter.
An endless photoconductor belt 26 is guided over a plurality of idler
rollers 27 to follow a path in the direction of arrow 22 to advance
successive portions of the photoconductive surface sequentially through
the various processing stations disposed about the path of movement
thereof.
The photoconductor belt 26 is driven by a drive rollers 101, driven with a
DC-motor with encoder feedback, the motor being coupled to the drive
roller 101 over a two-step reduction with a total reduction of 1/25. The
driving speed is kept constant by measuring the belt revolution time and
adjusting the speed so that the belt revolution time is constant. In this
manner a belt speed accuracy of 10.sup.-4 can be achieved.
Means (not shown) are provided controlling the lateral position of the
photoconductive belt 26.
The photoconductive belt may comprise a base layer of
polyethyleneterephthalate of 100 .mu.m thickness covered with a thin layer
of aluminum as a back electrode (less than 0.5 .mu.m thickness). The
organic photoconductor (OPC) layer is on top of the aluminum layer and is
from 15 .mu.m in thickness. To make contact with the aluminum back
electrode, the photoconductor has two strips of carbon/polymer mixture,
with a width of 10 mm, positioned beyond the image area and extending
through the OPC layer. Conductive grounding brushes (not shown) contact
these carbon strips. The belt is arranged such that the photoconductive
layer is positioned on the outside of the belt loop.
Initially, a portion of photoconductive belt 26 passes through charging
station 28. At the charging station, a corona-generating device
electrostatically charges the belt to a relatively high, substantially
uniform potential, the dark potential V.sub.0. Next, the belt passes to an
exposure station 29. The exposure station includes a raster output scanner
(ROS) 30 including a laser with a rotating polygonal mirror block which
creates the output printing image by laying out the image in a series of
horizontal scan lines. Exposure station 29 will expose the photoconductive
belt to successively record four latent color separation images. The
latent images are developed for example with magenta, cyan, yellow and
black developer material, respectively. These developed images are
transferred to the print sheet in superimposed registration with one
another to form a multicolor image on the sheet. The ROS receives its
input signal from an image processing system (IPS) 31. This system is an
electronic control device which prepares and manages the data inflow to
the scanner 30. A user interface (UI) 32 is in communication with the IPS
and enables the operator to control various operator-adjustable functions.
IPS 31 receives its signal from input 34. This input can be the output of
a raster input scanner (RIS), in which case the apparatus is a so-called
intelligent copier. In such case, the apparatus contains document
illumination lamps, optics, a mechanical scanning drive, and a
charge-coupled device. The RIS captures the entire original document and
converts it to a series of raster scan lines and measures a set of primary
color densities, i.e. red, green and blue densities at each point of the
original document. However, input 34 can as well receive an image signal
resulting from an operator operating an image processing station.
After an electrostatic latent image has been recorded on the
photoconductive belt 26, the belt 26 advances this image to the
development station. This station includes four individual developer units
35, 36, 37 and 38.
The developer units are of a type generally referred to in the art as
"magnetic brush development units". Developer units 35, 36 and 37,
respectively, apply toner particles of a specific color which corresponds
to the compliment of the specific color-separated electrostatic latent
image recorded on the photoconductive surface. The color of each of the
toner particles is adapted to absorb light within a preselected spectral
region of the electromagnetic wave spectrum. For example, an electrostatic
latent image formed by discharging the portions of charge on the
photoconductive belt corresponding to the green regions of the original
document will record the red and blue portions as areas of relatively high
charge density on photoconductive belt 26, while the green areas will be
reduced to a voltage level ineffective for development. The charged areas
are then made visible by having developer unit 35 apply green absorbing
(magenta) toner particles onto the electrostatic latent image recorded on
photoconductive belt 26. Similarly, a blue separation is developed by
developer unit 36 with blue absorbing (yellow) toner particles, while the
red separation is developed by developer unit 37 with red absorbing (cyan)
toner particles. Developer unit 38 contains black toner particles and may
be used to develop the electrostatic latent image formed from black
information or text, or to supplement the color developments. Each of the
developer units is moved into and out of an operative position. In the
operative position, the magnetic brush is closely adjacent to the
photoconductive belt, whereas in the non-operative position, the magnetic
brush is spaced therefrom. During development of each electrostatic latent
image only one developer unit is in the operative position, the remaining
developer units being in their non-operative one. This ensures that each
electrostatic latent image is developed with toner particles of the
appropriate color without inter-mingling. In FIG. 1, developer unit 35 is
shown in its operative position. Finally, each unit comprises a toner
hopper, such as hopper 39 shown for unit 35, for supplying fresh toner to
the developer which becomes progressively depleted by the development of
the electrostatic charge images.
Referring to FIG. 2, there is shown one of the developing units, namely
unit 35 which on its front side has a magnetic roller 51 consisting of a
non-ferromagnetic sleeve rotatable around a non-rotating magnetic core and
slightly protruding from the unit for bringing a layer of developer
adhering in the form of a brush to its outer surface into contact with the
photoconductive surface of the belt 26. The developing unit 35 is supplied
with magnetizable development material including non-permanently
magnetized magnetic carrier granules having toner particles adhering
triboelectrically thereto. The developer material is continually brought
through a directional flux field to form a brush of developer material.
The developer materials are continuously moving so as to provide the brush
consistently with developer material. The left hand part of FIG. 2 shows a
mixer arrangement 54 with a toner hopper 39, whereas the right hand part
is the driving mechanism 55 with inter-engaging gears for the driving of
the rotatable rollers of the unit 35. Magnetic roller 51 rotates in the
direction of the arrow 56 and the thickness of the layer of developer
supplied to its surface is metered by an adjustable doctor blade 57. The
representation of the toner hopper 39 is diagrammatic only, and it will be
understood that in practice the toner addition system will comprise a
toner cartridge or bottle suitably and removably connected to the unit,
and a metering system for feeding controlled amounts of toner to the unit
35.
Part of the development unit 35 is shown in cross-section in more detail in
FIG. 3. As will be seen in this Figure, the development unit includes a
magnetic roller 51. The moving photoconductive belt 26, moves close to,
but not in contact with, the magnetic roller 51. The distance between the
photoconductive surface of the belt 26 and the magnetic roller 51 is
constant and is determined by a fixed sliding backing shoe 53. A
controlled DC+AC potential is applied between the magnetic roller and the
back electrode of the photoconductive surface of the belt 26 via contact
brushes (not shown) by a control device generally represented at 52.
After their development, the toner images are moved to toner image transfer
stations 40, 41, 42 and 43 where they are transferred on a sheet of
support material, such as plain paper or a transparent film. At a transfer
station, a sheet follows the rectilinear path 17 into contact with
photoconductive belt 26. The sheet is advanced in synchronism with the
movement of the belt. After transfer of the four toner images, the belt
following an upward course is cleaned in a cleaning station 45 where a
rotatable fibrous brush or the like is maintained in contact with the
photoconductive belt 26 to remove residual toner particles remaining after
the transfer operation. Thereafter, lamp 46 illuminates the belt to remove
any residual charge remaining thereon prior to the start of the next
cycle.
The operation of the printer described hereinbefore is as follows.
The magenta latent image being exposed by station 29 on photoconductive
belt 26, this image is progressively developed by station 35 being in its
operative position as the belt moves therethrough. Upon completion of the
exposure of the magenta image, the yellow image becomes exposed. During
the yellow exposure, the developed magenta image is transported past
inactive stations 36, 37 and 38 while toner transfer stations 40 to 43 are
also still inoperative.
As the development of the magenta latent image is finished, magenta
development station 35 is withdrawn to its inoperative position and after
the trailing edge of the magenta image has passed yellow development
station 36, this station is put into the operative position to start the
development of the yellow latent image. While the latter portion of the
yellow latent image is being developed, the exposure of the cyan latent
image at 29 starts already.
The described processes of image-wise exposure and color development
continue until the four color separation images have been formed in
successive spaced relationship on the photoconductive belt.
A sheet which has been taken from stack 12 and kept in readiness in aligner
16, is then advanced and reaches toner transfer station 40 where at that
moment the last formed toner image, viz. the black one, is ready to enter
the station. Thus, the lastly formed toner image is the first to become
transferred to a sheet. The firstly formed toner image, viz. the magenta
one, takes with its leading edge a position on the belt as indicated by
the cross 62 and will thus be transferred last. The other two toner images
take positions with their leading edges as indicated by crosses 63 and 64,
respectively.
Thus, the timing of exposure of the four distinct images, the relative
position of these images on the photoconductive belt and the lengths of
the path of this belt between the successive transfer stations are such
that as a paper sheet follows a linear path through these stations, the
partly simultaneous transfer of the distinct toner images to the paper
sheet is such that a perfect registering of these images is obtained.
The sheet bearing a color toner image on its obverse side produced as
described hereinbefore, is now passed through image forming station 21 for
applying a color toner image to the reverse side of the sheet.
The buffer station 23 with an endless belt 24 transports the sheet bearing
the color images to the fuser station 25. The buffer station 23 allows the
speed of the sheet to change, thereby enabling the speed of fuser station
25 to be different from that of the speed of image forming stations 20,
21. In the apparatus according to the present embodiment, the speed of the
two photoconductive belts may be, for example, 125 or 250 mm/s, whereas
the fusing speed was 100 mm/s or less. The length of buffer station 23 is
sufficient for receiving the largest sheet size to be processed in the
apparatus. Buffer station 23 operates initially at the speed of the
photoconductive belts of image forming stations 20 and 21. The speed of
this station is reduced to the processing speed of fuser station 25 as the
trailing edge of the sheet leaves the second image forming station 21.
The fuser station 25 operates to melt the toner particles transferred to
the sheets in order to affix them. The fusing station 25 can be of known
construction, and can be arranged for radiation or flash fusing, for
fusing by convection and/or by pressure, etc. Hot fusing is preferred. The
fused sheet is finally received on platform 14.
EXAMPLES
Example 1
In this example, reversal development is used. A photoconductive belt was
charged to a dark potential of between 370 and 500 volts before being
exposed image-wise to create a charge image thereon. The belt was moved at
a speed of either 125 or 250 mm/sec past a development unit loaded with
commercially available DCP-1 developer containing 4.2% toner (ex Xeikon
NV). The development unit included a magnetic roller having a diameter of
20 mm, rotating at a circumferential speed which was twice that of the
linear belt speed. The magnetic roller was spaced at a distance of
0.65.+-.0.05 mm from the belt surface providing a development angle of
between 6.degree. and 8.degree.. The magnetic pole strength of the
development pole was 950.+-.50 Gauss. Developer was supplied to the
magnetic roller at between 65 and 80 mg/cm.sup.2. The AC bias was 1500
volts (peak-to-peak). After development of the image on the belt, the
toner image was transferred directly to a paper sheet substrate and the
product was examined for background development. Results were classified
as excellent (E), good (G), fair (F) and bad (B).
In the case of reversal development, the equation for Z can be re-written
as follows:
##EQU4##
where V.sub.0 is the dark potential in Volts, V.sub.DC is the DC bias
potential, f is the AC bias frequency in kHz, and v.sub.p is the speed of
the image forming belt in mm/s. The dark potential (V.sub.0 volts), the DC
bias potential (V.sub.DC volts) and the AC bias frequency (f kHz) were set
as given in the following Table 1.
TABLE 1
______________________________________
f V.sub.0 V.sub.DC
V.sub.p
(kHz) (volts) (volts) (mm/s) Result
Z
______________________________________
3 370 320 125 B 0.48
3 440 340 125 G 1.9
3 470 345 125 E 3.0
3 500 350 125 E 4.3
4 370 320 125 F 0.64
4 440 340 125 E 2.6
4 470 345 125 E 4.0
4 500 350 125 E 5.8
5 370 320 125 G 0.8
5 440 340 125 G 3.2
5 470 345 125 E 5.0
5 500 350 125 E 7.2
6 370 320 125 G 1.0
6 440 340 125 E 3.8
6 470 345 125 E 6.0
6 500 350 125 E 8.6
3 370 320 250 B 0.12
3 440 340 250 B 0.48
3 470 345 250 G 0.75
3 500 350 250 E 1.1
4 370 320 250 B 0.16
4 440 340 250 F 0.64
4 470 345 250 E 1.0
4 500 350 250 E 1.4
5 370 320 250 B 0.2
5 440 340 250 G 0.8
5 470 345 250 E 1.3
5 500 350 250 E 1.8
6 370 320 250 B 0.24
6 440 340 250 G 0.96
6 470 345 250 E 1.5
6 500 350 250 E 2.2
______________________________________
These results demonstrate that best results are obtained when the function
Z exceeds 0.65, especially when the function Z exceeds 0.8.
Example 2
This was similar to Example 1, except that the developer used was AG940 (ex
Agfa-Gevaert NV) containing 5% toner CB923. The results are set out in the
following Table 2.
TABLE 2
______________________________________
f V.sub.0 V.sub.DC
V.sub.p
(kHz) (volts) (volts) (mm/s) Result
Z
______________________________________
3 370 320 125 B 0.48
3 440 340 125 G 1.9
3 470 345 125 E 3.0
3 500 350 125 E 4.3
4 370 320 125 F 0.64
4 440 340 125 E 2.6
4 470 345 125 E 4.0
4 500 350 125 E 5.8
5 370 320 125 E 0.8
5 440 340 125 E 3.2
5 470 345 125 E 5.0
5 500 350 125 E 7.2
6 370 320 125 E 1.0
6 440 340 125 E 3.8
6 470 345 125 E 6.0
6 500 350 125 E 8.6
3 370 320 250 B 0.12
3 440 340 250 B 0.48
3 470 345 250 G 0.75
3 500 350 250 G 1.1
4 370 320 250 B 0.16
4 440 340 250 F 0.64
4 470 345 250 E 1.0
4 500 350 250 E 1.4
5 370 320 250 F 0.2
5 440 340 250 G 0.8
5 470 345 250 E 1.3
5 500 350 250 E 1.8
6 370 320 250 F 0.24
6 440 340 250 E 0.96
6 470 345 250 E 1.5
6 500 350 250 E 2.2
______________________________________
These results demonstrate that best results are obtained when the function
Z exceeds 0.65, especially when the function Z exceeds 0.8.
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